1. Field of the Invention
This invention relates to methods of geophysically prospecting by generating and detecting seismic waves and to a seismic wave generator which simultaneously generates compression and shear waves in a repeatable fashion.
2. Prior Art
Seismic surveying involves the generation of seismic waves in the earth by impacting the earth with a suitable generator, and the detection and analysis of the resulting waves at a remote point using a seismometer. The waves received by the seismometer are influenced by the subterranean geology and may be analyzed to determine the nature of that geology.
Prospecting of this type generally utilizes elastic waves involving earth particle movement in the direction of wave propogation. These waves, referred to as compressional or longitudinal waves, are readily generated by a vertical impact against the earth's surface by an explosion or a mechanical transducer. The technical literature has expressed the utility for seismic prospecting employing a second form of wave, termed shear waves, wherein the earth particle motion is generally normal to the direction of wave propogation. Shear waves have slower rates of propogation through the earth than do longitudinal waves so they produce a higher degree of revolution at a given frequency. This allows the detection of subterranean anomalies undetectable by the longitudinal waves and the detection of larger bodies with a higher degree of precision. Horizontally polarized shear waves are not converted into different wave types upon reflection or refraction from horizontal interfaces as is the case with compression waves and accordingly seismograms made from such waves are simpler to interpret.
Despite these recognized advantages, the use of shear waves in seismic prospecting has been greatly limited because of the unavailability of suitable shear wave generators. The difficulty encountered in designing such generators involves the manner of coupling an impact to the earth so that it will impart the desired shear motion to the earth's surface. The coupling device must prevent horizontal sliding motion of the force generator and convert the tendencies toward such motion into motion of the earth's surface. Previous shear wave generators have accomplished this attachment by using large masses to raise the friction between the generator and the earth or through the use of trenches, driven stakes or poured concrete slabs. These devices involved large and expensive static structures, incapable of being moved from site to site in the same manner as longitudinal wave generators, or have been capable of generating only very limited shear waves.